The present invention relates to a method and apparatus for detecting .sup.14 CO.sub.2 with a scintillation counter. The invention is especially useful for detecting the presence of biological activity and particularly for making rapid analyses of materials in which the presence of micro-organisms, or the like is suspected.
When, for example, bacteria are cultured in a suitable medium including a fermentable carbon source such as glucose, the source may be broken down to form CO.sub.2 during the growth of the bacteria. If the growth medium is alkaline the CO.sub.2 will generally be absorbed to form carbonates or bicarbonates. However, if the starting material is acidic (or slightly alkaline or neutral so that a slight amount of CO.sub.2 absorption will convert the same to acidic conditions), gaseous CO.sub.2 will be evolved into the atmosphere above the solution.
If the medium includes a carbon source prepared from radioactive carbon having an atomic weight of 14 (.sup.14 C) rather than normal carbon having atomic weight of 12, any CO.sub.2 liberated will include radioactive .sup.14 CO.sub.2. The radioactivity of the .sup.14 CO.sub.2 which is the result of beta decay of the .sup.14 C, can be used to activate a scintillation compound, causing it to emit light flashes. A scintillation counter can be used to measure these emissions which will be generally related to the amount of CO.sub.2 generated and thus can be used to measure the bacteria presence and growth in the medium.
Present methods for the detection and measurement of .sup.14 CO.sub.2 (See, for example H. N. Wagner, "Principles of Nuclear Medicine" p. 796-97 published by W.B. Saunders, Philadelphia) absorb or trap the .sup.14 CO.sub.2 in one operation and then transfer the absorber to a vial to which are added scintillating liquids. In a modification of this technique the absorber can be placed in a scintillation vial which is connected to the reaction flask by a conduit. These methods for determining bacteriological presence and growth by the radioactivity of the CO.sub.2 evolved from the medium are slow and laborious, and possess considerable chance for error. It has also been proposed to position a filter paper coated with a CO.sub.2 absorbing alkali salt and a scintillation compound between an open topped inner sample vial and an outer vial and measure the scintillations directly of this assembly. This system has a number of drawbacks. First the proposed inner vial is considerably smaller than the outer vial and therefore the amount of sample has to be small. Also the entire assembly has to be sterilized.
In hospital work, the early detection of bacteria and body fluids is of paramount importance. It has been a well recognized practice to place blood or urine specimens or the like in appropriate growth medium upon a Petri dish and make visual observations of bacteriological growth. While this method is also slow and laborious, it does facilitate the final identification of the bacteria. Manifestly, in each method, all samples, whether positive or negative for bacteria, had to be subjected to exhaustive procedures.
In many cases, a proper diagnosis and treatment of diseases would be facilitated if the absence of certain bacteria could be confirmed simply and quickly. Further, if such negative samples could be identified and discarded without extensive processing, much laboratory effort would be eliminated. The latter is also true if the presence of a certain species or at least of a certain group of bacteria could be established rapidly.
Another prior art method for determination of the presence of biological activity is described in U.S. Pat. No. 3,676,679, to Waters. In the system of this patent, the .sup.14 CO.sub.2 evolved is withdrawn from the reaction vessel and analyzed in gaseous form in an ionization chamber for the presence of radioactivity.